Shared forced air ducting in duplex

[Terry Beck]

1950 single family home, basement was converted to an apartment so it is now considered a duplex or multi-family dwelling. One forced air furnace in basement with shared HVAC ducting between units. I can find code that says cold return cannot be in another dwelling. There are rules about sheet metal requirements about ducting passing through habitable space.

But I can't a building code that prohibits shared HVAC ducting in a multi-family dwelling (duplex). I can find some code on fire wall separation between units in a townhome. Not sure if that applies to a duplex. Am I wrong that it is not allowed?

More info: I found a municipal directive from a city in CA regarding converting existing homes to a duplex:
"Houses greater than 5 years old where an existing furnace will be shared, require a duct-type smoke detector to be located in the supply or return air plenum in order to shut off the furnace if smoke is present."

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[Jerry Peck]

First, I'm not sure that I would call it a duplex, and I would definitely not call it a townhouse.

Look up the code definition of a townhouse.

I doubt that it meets the requirements for a two-family dwelling either.

You might best just refer to it as what it is: a single-family dwelling (also known as a one-family dwelling) with a basement converted into an apartment.

It may not have been done legally, and you don't want to be the one to 'imply that it may be' "legal".

If it us "legal", then there will be some type of code or ordinance that states what is required, and it seems like you already found part of it.

[Terry Beck]

This is a small college town (pop < 70,000), with a lot of turn-of-the century homes being used for student housing. The city does little or nothing about these beat up old fire traps until a major remodel is being done. The city seems to be relying on using zoning for in-fill construction to encourage tear-down rather than update. I'm amazed what people will live in for affordable rent.

This converted single family home has probably had this basement apt for 30 yrs or more. Only one breaker panel with the main disconnect in the basement apartment. Shared water heater. One gas meter. Furnace is an antique, and the buyers want to convert part of the building for commercial use (mixed use zoning). I will of course re-direct them to the city inspector, and the city may have directions for conversions like I mentioned above with allowances for converting existing home. Google shows several cities with documents that detail duplex conversion requirements, some much more forgiving than others.

I was just trying to establish a comparison as to what might be required if we looked at current safety standards. And I thought duct penetration was a key element.

[Jerry Peck]

I was just trying to establish a comparison as to what might be required if we looked at current safety standards. And I thought duct penetration was a key element.

With that being the guideline, then:

1 hour floor-ceiling assembly, with all required penetrations being 1 hr rated.

Membrane penetrations not to exceed the limitations of 100 square inches of penetration in 100 square feet of ceiling area, keeping in mind that "100 square feet" IS NOT an area which is "10 feet by 10 feet", "100 square feet could be an area which is 1 foot by 100 feet (it could also be an "L" shape or "T" shape area, the key is "area", no given shape), so you need to see how many things are in close alignment with each other, determine the overall width and length of that area, and calculate if that area is 100 sq ft.

Through penetrations need to have proper firestopping systems, and ducts would need fire-dampers as well as duct smoke detectors.

Those are just for starters.

Then there are proper egress and EERO issues to comply with, as well as natural light and ventilation ... once you start down this road ... the road gets rough and complicated and is full of potholes.

We stayed in a "townhouse apartment" for a week this summer while in Washington, D.C., and it was convoluted (as you described) - it was three story, we had the entire second floor of about 1300 sq feet, and there was an apartment on the third floor, a small apartment on the second floor where the main entry was, and two small basement apartments where the second entry was ... and I couldn't help but 'inspect it' every time we came in or went out ... I don't see how it would be allowed (if it was allowed) to be chopped up the way it was. I don't know about the other apartments, but the second floor apartment we stayed in was nice, three bedrooms, two bathrooms, kitchen, eating area, small balcony, living area (which served as the third bedroom with a Murphy bed) ... old structure probably from the 1930's or so, maybe older.

[Markus Keller]

Around here we call those illegal conversions and I write them up as such during an HI. Let the owner prove it isn't an illegal conversion. Which of course they won't be able to. The City has their own process for writing these up as illegal conversions and putting them in housing court for de-conversion.
From an HI standpoint its about informing the client about the potential liabilities if something goes wrong, potential costs for de-conversion and zoning. In municipalities that don't have an 'illegal basement or attic' basement conversion system set up I usually look up the zoning. If they are renting an apartment in a single family zoning district then they are violating the zoning.
There are multiple ways to address this issue. The important factor is that you inform the client so they know what the potential liabilities and costs are. Whether they leave it or deconvert is their business.